Semiconductor devices are ubiquitous in modern society and semiconductor manufacturers, for example manufacturers of solid state lighting devices, are constantly seeking to improve the performance of their products.
Recently, devices based on gallium nitride (GaN) have found a wide range of application. In particular, high brightness LEDs based on GaN/InGaN have been widely used, for example in backlighting of LCDs, traffic signals, full color displays and street lights. GaN/InGaN LEDs have also recently started to enter the general lighting market.
In order to be more effective in general lighting applications, the performance of InGaN/GaN LEDs has to be further improved. For instance, it is generally thought that the power conversion efficiency of GaN/InGaN LEDs during high power operation must be greatly increased (to at least 50%) in order for them to replace current fluorescent lamps (power conversion efficiency ˜20%) and provide the benefits of better consumer experience and cost effectiveness.
Typically, high power LED devices are grown on a sapphire substrate. Sapphire substrate-based LEDs have certain disadvantages which limit the degree to which their power conversion efficiency can be improved. Due to the electrically insulating nature and poor thermal conductivity (41.9 W/(m·K)) of the substrate, sapphire-based LEDs generally suffer from poor light extraction, poor thermal dissipation, high junction temperature (>100° C.) and large efficiency droop with increasing junction temperature (>40%). These drawbacks provide serious difficulties for further improving the LED efficiency under high power operating conditions.
To attempt to overcome some of those difficulties, the vertical LED concept has previously been proposed. The principle of the proposal is to remove the sapphire substrate and attach the LED to a substitute substrate which has good electrical and thermal conductivities. The substitute substrate serves as an electrode to conduct current, and as an effective heat dissipation path.
Previous methods of fabricating semiconductor devices have implemented a vertical device topology by various means. Typically prior art processes involve the process of final substrate dicing or scribing/cracking. Since in most cases the final substrates are made of metals, the dicing/scribing process may cause metal contamination of the LED devices. This may give rise to leakage current, and may cause device failure or reliability issues. For processes in which the whole LED wafer is attached to a metal final substrate, the mismatch between the LED wafer and the metal substrate may cause large stress generation and wafer bowing after the removal of the original growth substrate, thus potentially giving rise to device failure and reliability issues.
There may be a need for a method of fabricating semiconductor devices which can alleviate one or more of the above-mentioned difficulties, or at least provide a useful alternative.